docs: def get_thumbnail_source(self, obj):
          """
          Obtains the source image field for the thumbnail.
  
          :param obj: An object with a thumbnail_field defined.
          :return: Image field for thumbnail or None if not found.
          """
          if hasattr(self, 'thumbnail_field') and self.thumbnail_field:
              return resolve(obj, self.thumbnail_field)
  
          # try get_list_image, from ListableMixin
          if hasattr(obj, 'get_list_image'):
              return resolve(obj, "get_list_image")
  
          logger.warning('ThumbnailAdminMixin.thumbnail_field unspecified')
          return None